Chemical and Enzymatic changes affecting food quality

Enzymatic Changes

BrowningBrowning is the process of becoming brown, especially referring to food. Foods, including beverages, can turn brown through either enzymatic or non-enzymatic processes.

• Enzymatic browning is a chemical process, involving polyphenol oxidase, catechol oxidase and other enzymes that create melanins and benzoquinone from natural phenols, resulting in a brown color. Enzymatic browning generally requires exposure to oxygen, thus the browning that occurs when an apple, for example, is cut.

• Enzymatic browning can be beneficial for:1. Developing flavor in tea2. Developing color and flavor in dried fruit such

as figs and raisins.• Enzymatic browning is often detrimental to:1. Fresh fruit and vegetables,

including apples, potatoes and bananas2. Seafood such as shrimp

A variety of techniques exist for preventing enzymatic browning:1.Lemon juice and other acids lower the pH and remove the copper cofactor necessary for the responsible enzymes to function.

2.Blanching to denature enzymes and destroy responsible reactants.

3.Cold temperatures can also prevent enzymatic browning by reducing rate of reaction.

4.Inert gas, like nitrogen, prevent necessary oxygen from reacting.

5.Chemicals such as sodium bisulfite and citrates.

Some other chemicals used to prevent Enzymatic browning are:

Carboxylic acid.

Ascorbic acid and its derivatives.

Sulfur compounds.

Phenolic acid. others.

Acetic acid.Citric acid.Formic acid.Lactic acid.Malic acid.

Ascorbic acid.ascorbate.Erythorbic acid.Erythorbate. 

sulfites.sulfur dioxide.Cysteine.Methionine.Glutathione.histidine.   

Caeffic acid.Chlorogenic acid.Cinnamic acid.Coumatic acid.Feruric acid. 

4-hexyl resorcinol.Honey.Salt. (NaCl).

Ripening• Ripening is a process in fruits that causes them to become

more palatable. In general, a fruit becomes sweeter, less green, and softer as it ripens.

• Unripe fruits have not yet developed the sugars needed in order to be eaten. For example, green bananas are mostly made up of starch, much like a potato. During the ripening process, the starch in the banana changes into a sugar making it ready to eat. Green or unripe bananas, if stored at a temperature of 57 degrees F can be safely, ‘put to sleep’ meaning that the ripening process can be delayed without harming the fruit’s quality.

• Other changes that occur during ripening are:1. Change of colour2. Decrease in organic acid content of fruits3. Increase in soluble solids 4. Increase in total Sugar Content5. Decrease in Phenolic matter

Ilkay Tosun; N. Sule Ustun; Belkis Tekguler reported that the following changes occurred during the ripening of blackberry fruits:1. Colour became dark as the ripening progressed.2. Slight increase in the soluble solids content at the green and red ripening stages.3. Significant increase in soluble solids at the ripe stage.4. Increase in total sugar content.5.Continuous increase in water, soluble solids and sugar concentrations during fruit development.6. Titratable acidity increased during development,but was less in ripe fruits.7. Decline in organic acids.8. Phenolic matters decreased with ripening, whereas no significantdifferences were found between the green andred maturity stages.9.Increase in Anthocyanins.10. Potassium, copper, iron and manganese concentrations increased during the reddening period and decreased in the ripe mature stage.11. After ripening the calcium and magnesium accumulate in the cell wall and their concentrations increase during the fully mature stage.

Demethylation of Pectic Substances

• During fruit ripening, pectin is broken down by enzymes pectinase and pectinesterase. Pectinesterase catalyses the de-esterification of pectin into pectate and methanol. In this process the fruit becomes softer as the middle lamellae break down and cells become separated from each other.

• So, post-harvest demethylation of pectic substances in plant tissues

leads to softening of fruit during ripening.

Fermentation

• Fermentation is the generation of energy by catabolism of organic compounds with the help of microbes. Fermentation in simple terms is the chemical conversion of sugars into ethanol.

• Harvested crops decompose from the moment they are harvested due to attacks from microorganisms. These include:

1. Bacteria: Various bacteria can be responsible for the spoilage of food. When bacteria breaks down the food, acids and other waste products are created in the process. These waste products may be unpleasant to taste or may even be harmful to one's health. a typical example of such bacteria is alicyclobacilli.

Yeasts: Yeasts can be responsible for the decomposition of food with a high sugar content. The same effect is useful in the production of various types of foods such as bread, yogurt, cider, and alcoholic beverages.A number of methods of prevention can be used that can either totally prevent, delay, or otherwise reduce undesirable food fermentation. They include asepsis, refrigeration, freeing, use of preservatives,

canning, drying, irradiation, etc.

But in many cases, fermentation can be desirable also. Fermentation is used to produce alcoholic beverages such as wine, beer, and cider. Fermentation also is employed in the leavening of bread(CO2 produced by yeast activity) ; in preservation techniques to produce lactic acid in sour foods such as sauerkraut, dry sausages, kimchi, and yogurt; and in pickling of foods with vinegar (acetic acid).

Food fermentation has been said to serve five main purposes: 1.Enrichment of the diet through development of a diversity of flavors, aromas, and textures in food substrates.2.Preservation of substantial amounts of food through lactic acid, alcohol, acetic acid, and alkaline fermentations.3.Biological enrichment of food substrates with protein, essential amino acids, essential fatty acids, and vitamins.4.Elimination of antinutrients.5.A decrease in cooking time and fuel requirement

Post Harvest Senescence (aging)

• Crop senescence is the study of aging in Crops. Crops, just like other forms of organisms, seem to have both unintended and programmed aging.

• The hormones abscisic acid and ethylene are accepted by most scientists as the main causes of Senescence, but many scientists believe gibberellins and brassinosteroids are equally responsible. Cytokinins help to maintain the cell but when they are withdrawn or if the cell can not receive the cytokinin it may then undergo senescence.

Chemical Changes

Rancidity

• Rancidity, is the chemical decomposition of fats, oils and other lipids. When these processes occur in food, undesirable odors and flavors can result. In some cases, however, the flavors can be desirable (as in aged cheeses). Rancidity can also detract from the nutritional value of the food. Species of the Gram-negative bacterial rod Pseudomonas are major causes of rancidity.

• Three pathways for rancidity are recognized:

1. Microbial RancidityMicrobial rancidity refers to a process in which microorganisms, such as bacteria, use their enzymes such as lipases to break down fat. This pathway can be prevented by sterilization.

2.Hydrolytic RancidityHydrolytic rancidity occurs when water splits fatty acid chains away from the glycerol backbone in triglycerides (fats). The chemical term is ester hydrolysis. Usually this hydrolysis process goes unnoticed, since most fatty acids are odorless and tasteless. When, however, the triglyceride is derived from short chain fatty acids, the released carboxylic acid can confer strong flavors and odors.

3.Oxidative RancidityOxidative rancidity is associated with the degradation by oxygen in the air. Via a free radical process, the double bonds of an unsaturated fatty acid can undergo cleavage, releasing volatile aldehydes and ketones.

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Reducing Rancidification 1. Antioxidants are often added to fat-containing foods to delay the onset or slow the development of rancidity due to oxidation. Natural antioxidants include polyphenol (for instance flavonoids), ascorbic acid (vitamin C) and tocopherols (vitamin E). Synthetic antioxidants include butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), TBHQ, propyl gallate and ethoxyquin. The natural antioxidants tend to be short-lived, so synthetic antioxidants are used when a longer shelf-life is preferred. A combination of water-soluble and fat-soluble antioxidants is ideal, usually in the ratio of fat to water.

Natural Antioxidant mechanism vs. Synthetic antioxidant mechanism: Ascorbate typically reacts with oxidants of the reactive oxygen species. Such radicals are damaging to animals and plants at the molecular level due to their possible interaction with nucleic, proteins, and lipids. Sometimes these radicals initiate chain reactions. Ascorbate can terminate these chain radical reactions by electron transfer. Ascorbic acid is special because it can transfer a single electron, owing to the stability of its own radical ion called "semidehydroascorbate", dehydroascorbate. The net reaction is:RO • + C6H7O6- → ROH + C6H6O6• -The oxidized forms of ascorbate are relatively unreactive, and do not cause cellular damage.BHT behaves as a synthetic analogue of vitamin E, primarily acting as a terminating agent that suppresses autooxidation, a process whereby unsaturated (usually) organic compounds are attacked by atmospheric oxygen. BHT stops this autocatalytic reaction by converting peroxy radicals to hydroperoxides. It affects this function by donating a hydrogen atom:RO2. + ArOH → ROOH + ArO.RO2. + ArO. → Nonradical productswhere R is alkyl or aryl, and where ArOH is BHT or related Phenolic antioxidants. Each BHT consumes two peroxy radicals.

2. Rancidification can also be decreased by storing fats and oils in a cool, dark place with little exposure to oxygen or free radicals, since heat and light accelerate the rate of reaction of fats with oxygen. 3. The addition of antimicrobial agents can also delay or prevent rancidification by inhibiting the growth of bacteria or other micro-organisms.

Discoloration• Discoloration on the surface of foods can affect consumer

acceptance of these products. Discoloration is caused by a phenomenon known as Photodegradation.

• Photodegradation is degradation of a photodegradable molecule caused by the absorption of photons, particularly those wavelengths found in sunlight, such as infrared radiation, visible light, and ultraviolet light. Photodegradation includes photodissociation, the breakup of molecules into smaller pieces by photons. It also includes the change of a molecule's shape to make it irreversibly altered, such as the denaturing of proteins, and the addition of other atoms or molecules. This type of photodegradation is used by some drinking water and wastewater facilities to destroy pollutants.

• The light that is absorbed by the food can cause deteriorative reactions of the food constituents. In most solid foods, the light only penetrates the outer layer of the product and photodegradation occurs in this surface layer. Discoloration on the surface of foods can certainly affect consumer acceptance of these products.

• In liquid foods, light penetration can be greater and with mixing of the products due to agitation, larger portions of food constituents may be deteriorated.

The light sensitivity of a food depends on many factors including the: 1. light source strength and type of light that it emits 2. distance of the light source form the food 3. length of exposure 4. optical properties of the packaging materials 5. oxygen concentration of the food 6. temperature

Nonenzymatic Browning

Nonenzymatic, or oxidative, browning is a chemical process that produces a brown color in foods without the activity of enzymes. The two main forms of nonenzymatic browning are: 1.Caramelization 2.Maillard reaction.

Caramelization• Caramelization is the process of removal of water from a sugar

followed by isomerization and polymerization steps. Caramelization starts at relatively high temperatures as compared to the other browning reactions, and depends on the type of sugar.The process of caramelization involves following steps:1.Melting of the sugar at high temperatures.2.Condensation step. 3.Isomerization of aldoses to ketoses and further dehydration reactions.4.Last series of reactions include both fragmentation reactions (flavor production) and polymerization reactions (color production).

During caramelization several flavor components as well as polymeric caramels are produced. Caramels are complex mixture of various high molecular weight

components. They can be classified into three groups:

•Caramelans (C24H36O18)

•Caramelens (C36H50O25)

•Caramelins (C125H188O80)

2. The Maillard reaction is a chemical reaction between an amino acid and a reducing sugar, usually requiring the addition of heat. The sugar interacts with the amino acid, producing a variety of odors and flavors. The Maillard reaction is the basis of the flavoring industry, since the type of amino acid involved determines the resulting flavor; it also produces toast. Most of the effect of Maillard reaction, including the caramel aromas and golden brown colors, are desirable. But some of the effect of Maillard reaction, including foods darkness and off-flavor development, are undesirable.

Steps Condensation - amine/carbonyl: Reaction between reducing sugar and amino acid. Loss of water molecule produce imine able to cyclise result in N glycoside.

Rearrangement – enolization: Amadori rearrangement of ammonium ion. Alkali catalyzed isomerization reaction.

Fragmentation: Glycosyl amine and Amadori product are intermediates. In pH 4-7 Amadori product gives 1- & 3- deoxydicarbonyl compounds.

Strecker degradation: Reaction between aldehyde and aminoketone results in strong odor. Condensation of 2 aminoketone yield pyrazine, strong aroma compound.

Polymerization - brown color: Formation of brown nitrogen-containing pigments (melanoidins) by aldol condensation and carboyl-amine polymerization.

Respiration

• Metabolic activity in fresh fruits and vegetables continues for a short period after harvest. The energy required to sustain this activity comes from the respiration process. Respiration involves the oxidation of sugars to produce carbon dioxide, water and heat.

• The storage life of a commodity is influenced by its respiratory activity. By storing a commodity at low temperature, respiration is reduced and senescence is delayed, thus extending storage life. Proper control of the oxygen and carbon dioxide concentrations surrounding a commodity is also effective in reducing the rate of respiration.

References

1. Ilkay Tosun; N. Sule Ustun; Belkis Tekgul. Physical and chemical changes during ripening of blackberry fruits

2. Wikipedia.org3. http://www.foodnetworksolution.com4. http://www.food-info.net/uk/colour/enzymaticbrowning.htm 5. http://www.portofhueneme.org/connect_with_us/

educational_tours_docs/bananas.pdf6. http://www.scielo.br/pdf/sa/v65n1/12.pdf7. http://www.planthormones.info/ 8. Ivanova M., and Rost T.L., (1998). Cytokinins and the

Plant Cell Cycle: Problems and Pitfalls of Proving their function.

9.  "Margarines and Shortenings" by Ian P. Freeman